Zinc complexes catalytic hydrolysis studies

Bimetallic zinc complexes formed with hexaazamacrocycles were studied in the hydrolysis of activated carboxyesters. Potentiometric titration demonstrated the dominant presence of a dinuclear hydroxo bridged species at pH >7. /)-Nitrophenyl acetate is hydrolyzed with no loss of catalytic activity for at least 2.7 catalytic cycles 4... 

Breslow and coworkers420 have studied the hydrolysis of the anhydrides (128) and (129) in both the presence and absence of zinc(II) (and other metal ions). In the the absence of metal ion, hydrolysis of the anhydrides is independent of pH in the region 1.0-7.5, as also occurs with phthalic anhydride.421 However, in the presence of zinc(II), the hydrolysis is first order in hydroxide above pH 5. Table 28 lists values of kobs for the various substrates at pH 7.5 (for the metal complex k0bs = kOH[0H-]). The catalytic effects are of the order of 103. The pH dependence of the zinc(II) catalysis is consistent with attack by external hydroxide on a complex such as (130) where the metal acts as a Lewis acid catalyst. A further possibility involves attack by coordinated hydroxide on an uncoordinated anhydride carbonyl (131), and there is some evidence that this is indeed the process which does occur. 

The copper(II)-promoted hydrolysis of glycylglycine has been studied in some detail.120 Copper(II) ions catalyze the hydrolysis of glycylglycine in the pH range 3.5 to 6 at 85 °C.120 The pH rate profile has a maximum at pH 4.2, consistent with the view that the catalytically active species in the reaction is the carbonyl-bonded complex. The decrease in rate at higher pH is associated with the formation of a catalytically inactive complex produced by ionization of the peptide hydrogen atom. This view has subsequently been confirmed by other workers,121 in conjunction with an IR investigation of the structures of the copper(II) and zinc(II) complexes in D20 solution.122 Catalysis by cobalt(II),123 and zinc(II), nickel(II) and manganese(II) has also been studied.124-126... 

The above studies indicate that metal ions catalyze the hydrolysis of amides and peptides at pH values where the carbonyl-bonded species (25) is present. At higher pH values where deprotonated complexes (26) can be formed the hydrolysis is inhibited. These conclusions have been amply confirmed in subsequent studies involving inert cobalt(III) complexes (Section 61.4.2.2.2). Zinc(II)-promoted amide ionization is uncommon, and the first example of such a reaction was only reported in 1981.103 Zinc(II) does not inhibit the hydrolysis of glycylglycine at high pH, and amide deprotonation does not appear to occur at quite high pH values. Presumably this is one important reason for the widespread occurrence of zinc(Il) in metallopeptidases. Other metal ions such as copper(II) would induce amide deprotonation at relatively low pH values leading to catalytically inactive complexes. 

Tagaki et al. [24] and Fomasier et al. [25] reported another type of metallomicelle attached with a metal-bound alkoxide nucleophile. Tagaki s zinc(II) and copper(II) complexes (with possible structures 6a and b) promoted the hydrolysis of 4-nitrophenyl picolinate in a comicellar system with hexadecyl trimethy-lammonium bromide. However, no detailed mechanistic study was reported. Scrimin s zinc(II) and copper(II) complexes (proposed structures 7a and b) also promoted the hydrolysis of 4-nitrophenyl picolinate. A postulated mechanism for the catalytic activity of 7 is shown in Figure 4. An aggregate of 7 more effectively... 

In the recent literature, many examples of A/BPs containing benzophenones can be found. A first example concerns the study of HDACs. These enzymes catalyze the hydrolysis of acetylated lysine amine side chains in histones and are thus involved in the regulation of gene expression. There are approximately 20 human HDACs, which are divided into three classes (I, II, and III). Class I and II HDACs are zinc-dependent metallohydrolases that do not form a covalent bond with their substrates during their catalytic process, which is similar to MMPs. It has been found that hydroxamate 65 (SAHA, see Fig. 5) is a potent reversible inhibitor of class I and II HDACs. In 2007, Cravatt and coworkers reported the transformation of SAHA into an A/BP by installment of a benzophenone and an alkyne moiety, which resulted in SAHA-BPyne (66) [73]. They showed that the probe can be used for the covalent modification and enrichment of several class I and class II HDACs from complex proteomes in an activity-dependent manner. In addition, they identified several HDAC-associated proteins, possibly arising from the tight interaction with HDACs. Also, the probe was used to measure differences in HDAC content in human disease models. Later they reported the construction of a library of related probes and studied the differences in HDAC labeling [74], Their most... 

Stopped-flow fluorescence studies of ES complexes provided a direct comparison of the peptide binding aflBnities of the zinc and cadmium enzymes and, simultaneously, an explanation for the different roles of metals in peptide and ester hydrolysis (48). Cadmium carboxypeptidase binds the peptide Dns-(Gly)3-L-Phe as readily as does [(CPD)Zn] but catalyzes its hydrolysis at a rate that is reduced considerably (Figure 8). Initial rate studies of oligopeptides are in agreement with this observation. For all peptides examined, the catalytic rate constants of the cadmium enzyme are decreased markedly, but the association constants (1) (Km values) of the cadmium enzyme are identical to those of the zinc enzyme (48,51,57). However, in marked contrast, for all esters examined the catalytic rate constants of the cadmium enzyme are nearly the same as those of the zinc enzyme, but the association constants are decreased greatly. 

An important conclusion from these comparative studies is that mononuclear Zn(II) complexes can be as efficient for nitrocefin hydrolysis as binuclear systems, thus providing evidence that the second zinc center (Zn2) in metallo- 3-lactamases is not required for nucleophile activation or catalytic activity. 

The X-ray structural studies offer strong evidence in support of a Lewis acid catalytic role for the active site zinc ion in peptide hydrolysis. Since the carbonyl group is no doubt a much weaker dipole than HaO, the initial (ground-state) interaction between the zinc ion and the substrate carbonyl oxygen must be stabilized by the summation of the weak bonding forces between enz5mie and substrate. The result is to displace the transition-state of the enzyme-catalyzed reaction (relative to its hypothetical nonenzymatic cormterpart) along the reaction coordinate toward the enzyme-substrate complex. 

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